Fix broken book links and add section on adding new E2E tests

[salasberryfin]

/kind cleanup

What this PR does / why we need it:

Follow-up to restructuring the book #1353, as some links were found to be broken.

Additionally, this adds a new Developers section on basic information on how to add a new E2E test case to existing suite.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

NONE

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[cpanato]

cool
thanks

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cpanato, salasberryfin

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[salasberryfin]

/cherrypick release-1.8

[k8s-infra-cherrypick-robot]

@salasberryfin: #1358 failed to apply on top of branch "release-1.8":

Applying: docs: fix broken link after book restructure
Using index info to reconstruct a base tree...
A	docs/book/src/managed/index.md
Falling back to patching base and 3-way merge...
Auto-merging docs/book/src/topics/gke/index.md
CONFLICT (content): Merge conflict in docs/book/src/topics/gke/index.md
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config advice.mergeConflict false"
Patch failed at 0001 docs: fix broken link after book restructure

In response to this:

/cherrypick release-1.8

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